The bonding of two semiconductor components involves the accurate alignment of the components and the formation of electrical connections between multiple contacts on each of the components. These contacts are formed as metal areas embedded in a layer of dielectric, such as copper contact pads formed in oxide, and produced by a damascene-type process. In thermocompression bonding, contact bumps, such as microbumps formed of solder or formed of stacks of various materials (e.g. Cu/Ni/Sn), are produced on the Cu-contact pads prior to bonding, on one or both of the components that are to be bonded. In applications such as memory chips, dense arrays of contacts are to be interconnected, with the pitch of these arrays being of the order of 20 μm in present day technology. This can require a dense array of microbumps with a height and diameter of the order of 5-10 μm for example. One way of reducing the microbump pitch is to use embedded microbumps, e.g., embedding the microbumps in a planar layer of embedding material so that planar bonding surfaces are obtained. The embedding layer can enable a better alignment tolerance and a better control of the microbump height variation and of the microbump roughness. In addition, the microbumps are protected during process steps and manipulations of the components before the bonding step.
In some examples, a polymer can be used as the above-described embedding material for microbumps. “Surface planarization of Cu and CuNiSn Micro-bumps embedded in polymer for below 20 μm pitch 3DIC applications”, De Preter et al., 32nd Advanced Metallization Conference, 2015, describes the planarization of a polymer layer applied on an array of solder microbumps, by chemical mechanical polishing (CMP) or by a surface planer (so-called fly-cut technique). CMP and fly-cut are, however, expensive process steps that are not always available. CMP in particular may need to be optimized for each polymer type. For some polymers, it may be difficult or impossible to find a suitable CMP recipe.
In U.S. Patent Publication No. US2013/0299986, a method is proposed in which a layer of photopatternable polymer is applied on an array of solder bumps of a semiconductor die, and then selectively removed from the upper surface of the bumps by a lithography and etch process. The die is then bonded to another die by a thermocompression process, during which the polymer reflows and fills the complete area between the bonded dies and around the bumps. Bonding can be applied on wafer level, e.g., wafers comprising multiple dies are processed and bonded as described above. One drawback of this approach is that the polymer material together with the upper surface of the bumps may not form a uniform surface after the lithography and etch step. Especially when the bumps have a semi-spherical upper surface as shown in the drawings of US2013/0299986, the polymer layer does not closely adjoin the circumference of the bumps after lithography and etch, leaving an irregular upper surface of the die. Also, neither a uniform thickness of the polymer layer nor a uniform bump height across the full surface of a wafer comprising multiple dies can be guaranteed. Such non-uniformities may cause unwanted contamination of some of the bumps during the thermocompression step which can take place in a matter of seconds. Especially when important non-uniformities occur, this short timespan may be insufficient for the polymer to be distributed evenly between and around the bumps.